The dynamic range of a radio frequency (RF) receiver is typically limited by the noise floor of the RF receiver and the maximum signal that can be received without distortion. In certain measurements of signals output by a device under test (DUT), for example, such as Third Order Intermodulation (TOI) or Adjacent Channel Power Ratio (ACPR), the quantity of interest is the level of distortion present in the measured signal created by the DUT, and the quality of the measurement of an unknown device or signal is limited by the distortion created in the measuring receiver. Often, the distortion of the measuring receiver is larger than the distortion of the DUT signal. Conventional solutions for improving the distortion include adding attenuation in front of the receiver to reduce the maximum power of the DUT to such a level that the receiver distortion is significantly below that of the DUT signal. However, adding attenuation degrades the receiver noise figure, which may result in the DUT signal to be measured dropping below the noise floor of the receiver.
In a vector network analyzer (VNA), the upper limitation has been extended through use of gain compression compensation, as described for example in U.S. Pat. No. 7,231,308, “Test system dynamic range extension through compression compensation.” More particularly, U.S. Pat. No. 7,231,308 discloses a method to compensate for gain compression of the receiver through post processing the filtered data from the VNA receiver, which is created using an analog-to-digital converter (ADC) to sample the signal and a digital filter to extract magnitude and phase of the signal. While this method may be applied to single tone, continuous wave (CW) signals in a receiver, it does not provide compensation for multi-tone or wideband signals, in which several CW signals are present in the receiver at one time.
Thus, pre-distortion techniques for linearization of transmitters (e.g., DUTs) have been used for years, e.g., in the wireless base-station industry, to improve overall distortion performance of high power amplifiers. Receiver linearization correction, discussed above, has been successfully applied, for example to achieve improvement in gain compression performance. However, this implementation applies only to CW signals which occur on processed ADC readings, after decimation, filtering and detection. There is therefore a need for post-detection linearization techniques of the ADC output.